Neurofibromatosis type 1 (NF1) Rasopathy patients are predisposed to macrocephaly and to enlarged white matter tracts. NF1 patient brains also have abnormal bright spots on imaging, possibly representing abnormal myelin. We hypothesized that white matter alteration contributes to the cognitive, behavior and/or motor/sensory deficits that are common features of NF1 patients. We genetically engineered mice with Nf1 mutant brain oligodendrocytes and found that they have aberrant myelin and enlarged white matter tracts. Remarkably, altered signaling in oligodendrocytes also resulted in non-cell autonomous effects leading to increased permeability of the critical blood brain barrier. The Nf1 protein, neurofibromin, is an off signal for Ras GTPases, and mice with elevated Ras-GTP had the same cellular phenotypes as Nf1 mutants and were hyperactive. Downstream of Ras, loss of Nf1 in oligodendrocytes caused high reactive nitrogen species and most cellular abnormalities were reversed by anti-oxidant or nitric oxide synthase inhibition. These new models provide a unique opportunity to study oligodendrocyte effects on brain function, and regulation of the BBB. We plan a multi-faceted approach based on a collaborative team with specialized skills. In Aim 1, we will use a new flow cytometry (FACS) approach and mouse genetics to delineate signaling pathways that cause high ROS/NO in oligodendrocytes, because identified pathways should be relevant therapeutic targets in Rasopathy patients. In Aim 2, we will test if oligodendrocytes contribute to behavioral changes in NF1, as in the Ras mice we studied, and genetically define the critical NOS enzyme. In Aim 3, we will compare brains in which all brain cells or only oligodendrocytes have mutations in Nf1 using cell-type specific control of activity (optogenetics), advanced imaging (whole-brain functional MRI of awake mice) and electrophysiology. These studies will define aspects of brain structure, impulse conduction and changes at the level of brain cell populations in functional connectivity mediated by oligodendrocytes. They will also begin to identify how oligodendrocyte NO signaling contributes to cognitive and motor/sensory function, and to homeostasis of the blood brain barrier. We aim to provide preclinical support for human Rasopathy therapies that can be tested using imaging endpoints we identify.